(a) the total momentum must be conserved
(d) the total kinetic energy must change.
Explanation:
When the block moving in air breaks into two parts due to internal forces, the speeds of the parts will change. Since the Kinetic energy is proportional to the square of the speed it will be always positive. Hence the total kinetic energy must change.
The momentum is a vector and even if the velocities of the parts are different in magnitude and direction the vector addition of momentums will be same as before the split. Because in the absence of external force the total momentum is conserved.
